Note: Descriptions are shown in the official language in which they were submitted.
CA 02219066 1997-10-21
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PATENT
Docket No. 655.00558
HUMPED PLATE FIN HEAT EXCHANGER
Background of the Invention
The present invention is directed toward a plate fin heat exchanger, and more
particularly, to a humped plate fin utilized in such heat exchangers.
Background Art
Plate fin heat exchangers are well known. Generally they include a core
made up of a number of stacked plates spaced in a parallel relationship. The plates
have aligned holes through which tubes extend generally perpendicular to the plane
of the plates. The tubes are interconnected and carry a first fluid through the heat
exchanger. A second fluid, usually air, flows between the stacked plates. Heat
transfer occurs between these fluids by heat transfer through the fins and across the
tubes.
Increased heat transfer has been achieved by maximizing the surface area
of the plate fins exposed to the fluid surrounding the plate fins and by increasing the
turbulence of this fluid. This has been implemented by introducing indentations and
corrugations to a plate fin 10, as seen in Fig. 1. Figure 2 shows the prior art
corrugations 11. This manner of increasing surface area introduces a number of
drawbacks that may decrease plate fin performance. These drawbacks include the
increased flimsiness of the plate fin 10 in one plane due to the corrugations 11, the
increased susceptibility to damage during core construction, and the greater
likelihood of forming an uneven core. Each of these drawbacks can increase
production costs and/or decrease heat exchanger effficiency.
Another factor affecting heat exchange performance is the connection
between the tubes and fins. A tight tube-fin connection increases heat exchanger2 5 performance. A good tube to fin bond, such as a good soldered or brazed joint, is
therefore highly desirable.
In many plate fin heat exchangers, tubes 12 are pushed through aligned tube
holes 13 in the plates. Once in place, the tubes are mechanically expanded by
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Docket No. 655.00558
driving a so-called "bullet" or expanding mandrel through each tube. As a result, the
tube side walls are inelastically urged into close proximity to the surrounding fin
enabling the formation of an excellent bonded joint. Excellent heat transfer will then
exist across the fin-tube interface.
In some cases, however, tube expansion is impractical or even impossible.
For example, in prior art multiple row heat exchangers having hundreds of tubes 12,
it simply is not practical to expand the tubes because of the large number of them.
And when the tubes have dimpled surfaces or are otherwise provided with internalturbulators or strengthening webs, a bullet cannot be driven through them without
flattening out the dimples, destroying the turbulator effect they provide or breaking
the webs destroying the strength against internal pressure that they provide.
Consequently other solutions have been attempted to achieve the close proximity
necessary to assure a good brazed or soldered tube to a fin joint.
For example, prior art plate fin holes may be partially or wholly surrounded by
a collar 14. The prior art collars 14 shown in Fig. 3 are wrinkled where the collars 14
meet the fin 10. These wrinkles 15 prevent the collars 14 of the plate fin 10 from
making complete peripheral contact with the tubes 12, which can result in decreased
heat exchanger core performance as a result of the absence of solder or braze metal
where contact is lost.
For these and other reasons, the current state of heat exchanger performance
for a given size, weight and production cost is not totally satisfactory.
This invention is directed to overcome one or more of the above
problems.
Summary of the Invention
In one aspect of the present invention a plate fin heat exchanger is disclosed
having a plurality of tubes and plate fins, each plate fin having a plurality of arced
deformations extending in at least two spaced rows with a plurality of tube holes
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PATENT
Docket No. 655.00558
disposed therein. The plate fin also has a plurality of stiffening beads of trapezoidal
cross-section disposed between the arced deformations.
It is an object of the invention to provide a heat exchanger that can be
substituted for a prior art heat exchanger of a given size, and have greater heat
s transfer performance than the prior art unit.
It is also an object of the invention to provide a heat exchanger of a given size
and performance level having a lower weight than the substitutable prior art heat
exchanger.
It is a further object of the invention to provide a plate fin heat exchanger
wherein the collars surrounding the tube holes of the plate fins have less wrinkling
than the prior art plate fins.
It is also a further object of the invention to provide a manufacturer with a
variety of choices of new core constnuction to replace cores constructed of the prior
art plate fins.
It is a still further object of this invention to provide a plate fin heat exchanger
constructed of plate fins having an increased surface area without suffering a loss
of fin stiffness.
Brief Description of the Drawings
Figure 1 is a plan view of a commonly used prior art plate fin.
2 o Figure 2 is a cross-sectional view approximately along the line 2-2 in Fig. 1.
Figure 3 is a cross-sectional view approximately along the line 3-3 in Fig. 1.
Figure 4 is a view of a heat exchanger core made according to the invention.
Figure 5 is a plan view of a plate fin made according to the invention.
Figure 6 is a cross-sectional view of the line 6-6 in Fig. 5.
2 5 Figure 7 is a cross-sectional view of the line 7-7 in Fig. 5.
Figure 8 is an enlargement of one collar as shown in Fig. 6.
Figure 9 is a cross-sectional view approximately along the line 9-9 in Fig. 5.
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PATENT
Docket No. 655.00558
Figure 10 is a graph comparing the overall heat exchanger performance of a
variety of cores as the number of fins-per-inch vary, with water flowing through the
tubes.
Figure 11 depicts the same comparison as Fig. 10 for a 50t50 ethylene
glycol/water mixture at a first flow rate.
Figure 12 depicts the same comparison as Figs. 10 and 11 for a 50/50
ethylene glycol/water mixture at a second flow rate.
Figure 13 is a fragmented plan view of a dimpled tube.
Description of the Preferred Embodiment
It is to be understood that the present invention is not limited to the particular
heat exchanger set forth below, and that the dimensions set forth below are for
purposes of illustration and enablement only.
One embodiment of a heat exchanger 16 contemplated by the current
invention is shown in Fig. 4 and has a core which includes a plurality of tubes 18
extending through a number of stacked plate fins 20. The tubes 18 are placed in
communication with each other by headers and tanks (not shown) to form a pathwaythrough the tubes 18 having an inlet which receives the first fluid from a source and
an outlet which delivers the first fluid from the tubes 18 to a destination outside the
heat exchanger.
In one embodiment, the tubes 18 have a major dimension of 0.625" (5/8") and
a minor dimension of 0.076" and can be smooth tubes or turbulated tubes with
0.014" high dimples. However, those skilled in the art will readily recognize that
other dimensions may be used as desired. The tubes 18 are parallel to each otherand extend through several stacked plate fins 20 generally perpendicular thereto.
2 5 The tubes 18 will typically have dimples (not shown) in their side walls. The dimples
extend toward the center of the tube and induce turbulence in the first fluid flowing
therein. The increased turbulence, of course, improves heat transfer as is well
known. It should be recognized, however, that plain tubes, that is, tubes without
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PATENT
Docket No. 655.00558
dimples, may be used as well and are specifically contemplated for use in one form
of the invention.
The plate fins are humped plate fins 20 and are made of copper sheeting,
approximately 0.003" thick, and have several arced deformations 22 aligned in
equally spaced rows 24 extending across the entire plate fin 20 surface (Fig.5). The
arced deformations 22 are humps formed by a rolling and/or stamping process, andhave a 0.3125" radius to a center point and a high-point 0.076" above the plane of
the plate fin 20 (Fig. 6).
The tube holes 28 are disposed at regular intervals within the arced rows 24.
The tube holes 28 are spaced 0.3853" apart, and are sized similar to the
corresponding tubes 18 to ensure a tight fit. In Fig. 5, each tube hole 28 has a major
dimension measuring 0.6300 + 0.0020" and a minor dimension measuring 0.080 +
0.0020". The plate fin - tube connection is a tight fit, wherein a collar 30 of the plate
fin 20 is substantially flush to the tube 18. That is to say, peripheral contact of each
tube 18 within hole 28 and the collar 30 is desired.
The tube holes 28 are formed by rolling a stamping die along the plate fin 20
to stamp a tube hole 28 and a surrounding collar 30 as shown in Fig. 6. During the
stamping process, a portion of plate fin 20 is bent from the plane of the plate fin 20
and acts as the collar 30. The collar 30 is essentially wrinkle-free and extends along
all sides of the opening 28. Along the opening's major axis sides, the collar 30follows the contour of the arced row 24, as shown in Fig. 8. The minor axis portion
31 of the collar 30 extends downward from the plane of the plate fin 20 in a generally
triangular shape, substantially perpendicular to the general plane of the plate fin 20,
as shown in Fig. 9.
A series of pyramidal shaped stiffening beads of trapezoidal cross section are
disposed between the arced rows 24 in the plate fin 20. Short stiffening beads 42
and long stiffening beads 44 are disposed in rows 40 between the arced rows 24 and
extend above the plate fin 20 plane 0.0160 + 0.0020". Short stiffening beads 42
have a 0.0880 x 0.2473" rectangular base and a 0.1993" x 0.0400" cap. Long
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PATENT
Docket No. 655.00558
stiffening beads 44 have a 0.3389" x 0.0780" base and a 0.2909" x 0.0300" cap.
Both long and short stiffening beads, 42 and 44, are laid out in rows 40 between the
arced rows 24 (Fig. 7). The long stiffening beads 44 extend lengthwise parallel to
the major axis of the tube holes 18. The short stiffening beads 42 are disposed
perpendicular to and between the long stiffening beads 44.
The tubes 18 are inserted through the plate fin 20 tube holes 28 as follows.
First, several plate fins 20 are placed in a fin jig which holds them during core
construction. The fins 20 are aligned such that corresponding tube holes 28 are
aligned. Next, tubes 18 are pushed through the aligned tube holes 28 and inserted
from the convex side of the humped fin. Due to the above-described sizing of thetube holes 28 and the tubes 18, a tight fit is obtained at the tube-plate fin connection.
Forming the collars 30 around tube holes 28 set within the arced deformations 22provides collars 30 that are substantially wrinkle-free. This allows the collar 30 to be
disposed in continuous abutment with the tubes 18. This connection can increase
heat exchanger core stability and improve heat exchange performance of cores
having this construction.
The improved heat transfer performance of the heat exchanger cores
contemplated by this invention has been verified by computer heat transfer models
and test results. The graphs in figures 10-12 compare the core performance of heat
exchangers having prior art plate fins (Fig. 1) with those having humped plate fins
20 herein described (Fig. 5). Specifically, each graph compares the heat exchange
performance of a heat exchanger constructed of a prior art seven-tube-row plate fin
(curve A) with heat exchangers having four and five tube-row humped plate fins 20.
The heat exchangers utilizing humped plate fins 20 had both plain tubes (PT) anddimpJed tubes (DT) and are as follows:
Curve Heat Exchanger Contours
B four tube row, plain tube
C five tube row, plain tube
D four tube row, dimpled tube
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PATENT
Docket No. 655.00558
E five tube row, dimpled tube
Computer generated data points are shown as an "O" whereas data points taken
from actual test data are shown by an "X".
Heat exchange performance is charted in Figs. 10-12 in quality control
btu(QCBTU). The QCBTU figure is obtained by adding together the amount of heat
rejected at the operating point for each of three standard fan curves. The amount
of heat rejected is based on an entering temperature potential of 100~F where
potential is defined as the difference between the average coolant temperature and
the entering air temperature. The resulting QCBTU is a single figure representing
an overall performance of the core and is expressed in BTU/min/FY face area at
100~F potential. The type of fluid and the total fluid flow rate must be the same for
each core type being compared.
It should be noted that for any given number of tube rows 24 and fins per
inch (FPI), the heat transfer performance of cores having the humped plate fin
element 20 exceeds the heat transfer performance of cores constructed with the
prior art fin element 10. Additionally, as the number of fins per inch increases, the
heat transfer performance of cores made with either fin increases. As the fins per
inch numbers increase, the cores having the improved humped plate fin 20
construction show an increase in heat exchange performance of a greater rate than
those having the prior art (Fig. 1 ) construction.
The data shows that the present humped plate fin element 20 achieves a
higher heat transfer performance than prior art plate fins 10 at any given core
configuration.
Further, Figs. 10-12 show that at high water flow rate, the use of dimpled
2~ tubes improves performance slightly. Figure 13 shows a flattened tube 12 having
dimples 50 in one side and dimples 52 in the opposite side wall. The dimples 50 and
52 are concave to the exterior of the tubes. Moreover, the dimples 50 in one side
wall are staggered with respect to the dimples 52 in the other side wall to force the
heat exchange fluid within the tubes to follow a tortious path and to increase
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PATENT
Docket No. 655.00558
turbulence. However, when 50/50 ethylene glycol/water is used as the coolant,
performance is increased substantially, especially at lower flow rates, by the use of
dimpled tubes. These conclusions hold for whatever fin/tube combinations are used
for the radiator.
These curves show that the manufacturer has several choices open to him
when replacing a prior art radiator core with a core constructed of the present
humped plate fins 20 to achieve the same or better performance. For example fromFigure 1 1, an 1 1 fins per inch prior art core having a flow rate of 192 Ibs. per minute
50/50 ethylene glycol/water can be replaced with a 9 fins per inch 4 row plain tube
core or a 7 fin per inch 5 row plain tube core. If a dimpled tube is used, both the
number of fins per inch and number of tube rows could be further reduced. The
resulting core would be thinner than the prior art core and would weigh less. It is
also believed that production and transportation costs would be reduced.
From the foregoing it will be appreciated that a heat exchanger made up of
a humped plate fins of the current invention offers many benefits over the prior art.
First, the heat exchanger with a humped fin construction can be substituted for a
prior art heat exchanger of the same size and weight and offer greater heat transfer
performance than the prior art unit. Also, a humped fin heat exchanger with a given
heat exchanger performance level will have a lower weight than an equally well
performing prior art heat exchanger. Further, because the humped plate fin
construction utilizes stiffening beads and not corrugations extending across the plate
fin, the humped plate fin offers greater stability and stiffness than does the prior art
plate fin. This attribute decreases core defects and delays that occur during heat
exchanger
construction. These stiffening beads may also increase the turbulence of the second
fluid.
The foregoing disclosure of specific embodiments is intended to be illustrative
of the broad concepts comprehended by the invention.
